Microwave point instruments can be used in liquid or bulk material applications. The microwave instrument provides detection of level at the installation point or change of products inside a liquid or bulk material storage and process vessels. There are no moving parts in contact with the medium.
A known microwave point level switch in one form utilizes a microwave generator periodically developing a pulse of microwave energy. The energy is directed to a waveguide configured as an electromagnetic bridge. The microwave pulse is divided into a measurement arm and reference arm. The reference arm includes tunable reference means for altering the microwave energy using a variable impedance tuning circuit. The measurement arm includes a process seal disposed in the vessel at a position at which the material is to be detected. The dielectric properties of the material in the vessel alter the microwave energy delivered thereto. In a waveguide configured as the well-known "magic-T" the altered signals from the reference arm and measurement arm are combined to develop a microwave signal in a detector arm equal to the difference between the altered microwaves. The magic-T bridge is calibrated for a select product by using the variable impedance tuning circuit to provide a null output when balanced. A sensing circuit coupled to the detector arm determines if the system is out of balance by sensing voltage changes.
As with any process instrument, testing of the instrument is important. While instruments often include means for testing circuits, it is often difficult to test the entire instrument, including the sensing apparatus. One known magic-T based microwave level switch uses a test port openings in the reference arm. A small tool such as a screwdriver can be placed in the test port to simulate change in process conditions. While this can be an effective test, the test requires that the instrument housing be opened to access the waveguide. Practically, this cannot be done on line or with any regularity.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.